Editing Thyroid (section)

== '''Related nerves''' ==

=== Autonomic ===

** Directly supplied by a network of tiny autonomic nerves from the superior and middle cervical sympathetic ganglia, and parasympathetic fibres from the vagus

=== '''Recurrent laryngeal nerve''' ===

** Embryology
*** Derived from the 6th pharyngeal arches, which form below the 6th aortic arches on each side
*** The nerves hook around 6th aortic arch structures and are pulled caudally during development
*** Parts of those arches then regress, leading to the final position of the nerve
*** Both nerves loop back, or 'recur', into the neck due to the heart and great vessels descending into the thorax, bringing the RLNs down with them
**** Left: The distal 6th arch persists as ductus arteriosum, and the 5th arch regresses. RLN loops under ligamentum arteriosum at the aortic arch and travels in the trache-oesophageal groove until it reaches the thyroid
**** Right: The distal 6th aortic arch and entire 5th arch regress, but the 4th arch persists as subclavian artery. RLN loops under the right carotid-subclavian artery junction  and migrates to the cricothyroid joint at the insertion into the larynx. It is therefore found in a slightly anterior plane and an oblique direction compared to the left RLN, which tends to stay deeper and straighter in the trache-oesophageal groove.

* Function
** Most important nerve to the larynx
** Innervates the motor function of all the intrinsic laryngeal muscles except for the cricothyroid
** Sensory fibres from the lower larynx
** Minor motor and sensory fibres from the trachea and oesophagus
* Path
** Right:
*** From vagus nerve as it passes over right subclavian
*** Loops posteriorly under the artery, emerging medially to the right CCA in the central neck
*** Travels either within or in close proximity to the trach-oesophageal groove, along a slightly oblique course from lateral to medial (because it is pushed more laterally in the chest), and slightly more anterior to the left RLN
*** Can lie either in front of or behind the ITA
** Left
*** Arises from left vagus as it crosses the aortic arch
*** Hooks posteriorly below the vessel, to the left and behind ligamentum arteriosum
*** Ascends on the right of the arch
*** Enters the trache-oesophageal groove, posterior to pre-tracheal fascia, and passes vertically
*** Most likely lies behind the ITA at the level of the thyroid
** Both
*** Often makes a small 'knee' just prior to inserting to the larynx, which is the most common site for nerve injury
*** Dives into larynx beneath the inferior-most fibres of cricopharyngeus (inferior constrictor), behind the inferior cornu of the thyroid cartilage
* '''Landmarks'''
** Characteristic glistening whitish colour compared to adjacent vessels; often has tiny blood vessels running along its surface (vasa nervorum).
** Look first at mid-thyroid level, travelling within or in close proximity to the '''trache-oesophageal groove''' (60% of cases)
** '''Tubercle of Zuckerkandl''' - the nerve is posterior to the tubercle in 90% of cases
** '''Berry ligament/inferior thyroid artery''' are typically intimately associated with the nerve at the level of the cricoid cartilage. The nerve crosses the artery, usually posteriorly, and typically curves anteriorly toward the ligament before diving posteriorly again into the laryngeal insertion point at the cricothyroid joint (100% of nerves are posterior to ligament of Berry)
** RLN always behind the ligament of Berry
** Typically crosses the inferior thyroid artery perpendicularly as it travels into the larynx
** RLN enters the larynx approximately 1cm below and just anterior to the readily palpable inferior cornu of the thyroid cartilage, just inferior to the lower edge of the inferior pharyngeal constrictor muscle (the most consistent RLN anatomical location in the neck)
* Variations
** RLN may branch more proximally in 20-30% of cases - need to preserve all branches, especially the anterior branches, which predominantly provide motor function.
*** Typically occurs in the last 2cm of RLN
** Non-recurrent laryngeal nerve -
*** Right (1%) - associated with aberrant right subclavian artery arising directly from the distal aortic arch instead of the innominate artery (the 'lusoria artery'). Right RLN then follows a straight path from vagus to larynx. If this condition is present, the right subclavian will pass posterior to oesophagus. Operative clue is that the carotid on that side is medial.
*** Left (extremely rare) - associated with situs inversus and a right-sided aortic arch
* Injury
** Unilateral - paralysis of the ipsilateral vocal fold. Symptoms range from voice complaints such as hoarseness and vocal fatigue, to aspiration
*** I think if EBSLN is ok, the cord lies close to midline, and a lot of voice compensation from the other side can occur
*** If EBSLN is also damaged, the cord will be in mid-adduction, and hoarseness/inability to cough will be worse
** Bilateral - bilateral vocal fold paralysis
*** Median position - prevent adequate air exchange - may require tracheostomy
*** Lateral position - high-risk for recurrent aspiration and pneumonia

=== '''External branch of superior laryngeal nerve''' ===

** Anatomy
*** SLNs branch off vagus at the level of hyoid bone and run along the inferior pharyngeal constrictor, before running parallel to the upper aspect of the superior pole thyroid vessels and then terminating in the cricothyroid muscle
*** EBSLN typically runs fairly high above the thyroid lobe
*** Take care when ligating and dividing the superior pole vessels - can sometimes run quite close to the vessels and the upper thyroid lobe, and sometimes need to dissect it away
*** Cernea classification below
** Function
*** Larynx - cricothyroid muscles, and contributes to vocal fold tone and tension
** Injury
*** Paresis of ipsilateral cricothyroid, leading to inability to tighten ipsilateral cord
*** Difficulty achieving high pitch and vocal projection and volume
**

=== '''Internal branch of SLN''' ===

** Supplies sensation to larynx above vocal cords

=== '''Cutaneous nerves of neck''' - see 'neck' ===